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has been rising at about 25% per year, compounded, since then. As a consequence, overall in the period 1995-2005, it rose 12-fold. As of 2005, about 60,000 megawatts (MW) of wind power had been installed. Large growth in wind-power installations has occurred in Germany (currently the world leader in wind power, with 40% of installed power), Spain, the United States (which was the leader in the 1990s), India, and Denmark (which generates more than one-fifth of its electricity this way). This technology could be useful in many other parts of the world as well. A 2003 EU report predicts that 4% of the world's energy will be produced by wind power in 2030.

Technically, six times the 2001 world electricity output could be produced from wind, but only 0.5% was actually produced globally in this way in 2005, although 3% of Europe's electricity was produced in this way. A landmass the size of China would be needed to satisfy world electricity demand from wind alone. More realistically, wind power could be expanded to provide up to perhaps one-fifth of the world's electricity.

If price is not taken into account, then the country with the highest potential for wind power is the United States. About 90% of the U.S. potential for wind power lies in 12 states in the Midwest, ranging from North Dakota to northern Texas (see Figure 8-3), though the demand for electricity is centered far from most of these areas. Indeed, the United States has enough potential wind power to supply all its electricity now and in the foreseeable future. The world's largest wind farm covers 130 km2 in Oregon and Washington and will eventually involve 460 turbines.

http://rredc.nrel.gov/ wind/pubs/atlas/maps/ chap2/2-10m.html.]"/>
FIGURE 8-3 Percentage of land area estimated to have class 3 or higher wind power in the contiguous United States. [Source: "Wind Energy Resource Atlas of the United States," Chapter 2; http://rredc.nrel.gov/ wind/pubs/atlas/maps/ chap2/2-10m.html.]

Wind Speed and Windmill Size

As one would intuitively expect, the greater the velocity, <u, of the wind, the greater the amount of energy a windmill or wind turbine will produce. In fact, the energy increases very sharply with wind speed. The energy yield from wind is proportional to i.e., to the third power of the wind speed. Consequently, a small improvement in velocity produces a large increase in yield; e.g., an increase from 22 to 26 mph improves the energy yield by two-thirds!

The cubic dependence of energy on wind speed is the result of two factors. First, the kinetic energy of the motion of the air mass in the direction of the wind is proportional to the square of the air speed, since from physics we know that for any moving body, its kinetic energy is given by mv2/2. Second, the amount of wind passing over the blades per unit of time increases linearly in direct proportion to the wind speed. The energy available to the wind turbine is equal to the product of these two factors, so it is proportional to qA Hence, much of the energy available to windmills occurs in short bursts of high velocity due to this strong dependence of energy on wind speed.

The energy the windmill can gather is proportional to the square of its blade length, since the area the blade sweeps out is proportional to the length squared. Since wind speeds increase with height above the ground, a tall turbine is also more efficient for this reason.

Each windmill in a wind farm extracts energy from the flow of air on it, so the individual windmills must be physically separated from each other to some extent. For technical reasons, no more than about one-third of the energy passing by a windmill can be extracted from the flow of air around it.

Potential Wind-Energy Sites

As a consequence of the local terrain, some geographical regions experience almost constant windy conditions. Geographical areas are commonly divided into seven classes of potential wind-power density, with class 7 having the highest potential. Ideal locations for wind farms are those with an almost constant flow of nonturbulent winds in all seasons. Although wind energy does rise steeply with wind velocity, locations with sudden gusts of high-speed wind are not considered favorable. Locations at less than 2-km altitude, with average wind speeds of at least 5 m/sec, corresponding to 18 km/hr (11 mph), are generally required for a location to be considered economically feasible. Some authors use the criterion of annual mean wind speeds 2:6.9 m/sec (25 km/hr or 15 mph) measured at 80 m, the tip of the blade height of modern windmills, as suitable for low-cost wind-power generation. Such sites are considered to be of class 3 or higher potential wind-energy sites. The regions of high wind-power potential at reasonable cost are the United States, Canada, South America, the European countries that are members of the Organization for Economic Cooperation and Development (OECD), and the former Soviet Union. The areas with the lowest potential are in Africa, eastern Europe, and Southeast Asia. In most areas, the potential exceeds the current electricity usage.

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